How often to sample for Legionella in the UK

Here is the part that catches people out: for most hot and cold water systems, UK guidance does not give you a sampling frequency at all. There is no rule that says “test every outlet quarterly”. Whether you sample, and how often, falls out of your risk assessment and the way you actually control the system [1][2].

That is not a dodge. It is the whole point. Sampling is verification, not control, and how much verification you need depends on what you are controlling and how confident you are that it is working.

So the question worth asking is not “what’s the frequency?” but “what does my system need me to prove, and how often?” Below is how to answer that for a real building, with a checklist you can take onto site.

When sampling is even on the table

Separate your systems first, because the guidance treats them very differently.

Most domestic-style hot and cold water systems are controlled by temperature: hot water kept hot, cold water kept cold, water kept moving. Where that control is in place and your monitoring shows it holding, routine microbiological sampling is generally not expected as a matter of course [2][3]. Chasing quarterly Legionella counts on a well-run temperature-controlled system rarely tells you anything your temperature records did not already say.

Sampling earns its place in narrower situations:

Where control relies on something other than temperature, such as a water treatment or biocide programme, sampling becomes a meaningful check that the treatment is doing its job, and more regular sampling may be advised [2].
Where you cannot reliably hold the right temperatures, sampling helps you understand the residual risk while the underlying fault is put right.
Cooling towers and evaporative condensers are a different category. Routine sampling at regular intervals is a normal expectation here, not an optional extra [2].
Spa pools sit under their own guidance, HSG282, with sampling expectations distinct from the hot-and-cold rules [5].
After commissioning, after remedial works or disinfection, and during an outbreak investigation, sampling steps up regardless of the routine pattern [4].

The frequency, in other words, sits downstream of the system type and the control method. Settle those before you argue about a number.

A checklist for setting your sampling frequency

Work through this on site with the risk assessment open in front of you. It is built to produce a written, defensible decision, not just a figure.

Establish what you’re controlling

Identify what actually controls each system: temperature, a treatment programme, or a mix of both.
Note where temperature control is genuinely reliable and where the records show it slipping.
Flag the higher-risk systems: cooling towers, spa pools, anything throwing aerosol near vulnerable people.

Decide whether routine sampling is warranted

For temperature-controlled systems holding their values, confirm whether routine sampling adds anything beyond your existing monitoring.
For systems controlled by other means, set sampling as the check on that control.
For cooling towers and spa pools, set a routine interval in line with the relevant guidance.

Set the interval and the triggers

Write down the routine interval the risk assessment supports for each system you sample.
List the events that raise it: temperature excursions, a positive result, remedial works, commissioning, a change in use or occupancy, an investigation.
Define what a result above the action level triggers, and name who owns that response.

Make the sample worth taking

Use a competent sampler and methods consistent with BS 7592 for site selection and technique [4].
Send samples to a UKAS-accredited laboratory.
Record where each sample was taken and why, so the next person can repeat it exactly.

Close the loop

Log the decision, the interval, the responsible person and the review date.
Revisit the schedule when the system, its use, or the control evidence changes.

Turning the checklist into a record that holds up

A frequency decision is only as good as the reasoning written next to it. “Quarterly” on its own means nothing in two years. “Quarterly samples at the cooling tower because control is by biocide dosing; raised to monthly after any dosing failure; reviewed annually” tells the next responsible person, or an inspector, exactly why the number is what it is.

Record the negative results too, and the in-range temperatures behind them. A clean sample sitting on top of a run of correct temperatures is genuine evidence of control. A clean sample sitting on top of months of missed temperature checks is close to meaningless, and that is exactly the kind of false comfort that gets organisations into trouble. If your records live in a binder nobody opens, the move to searchable logs is worth the effort: see Case study: improved compliance with digital logbooks.

The bits a frequency table won’t tell you

Two things get skipped almost every time.

First, the sample point matters as much as the interval. A sample drawn from a busy outlet that runs constantly will look fine and prove nothing about the dead leg upstairs. Frequency without representative sampling locations is just tidy paperwork.

Second, a sampling schedule is not a substitute for fixing known problems. If you already know an outlet runs tepid, or sits unused for weeks, the answer is to correct the temperature or the stagnation, not to sample it more often and hope. Sampling can tell you that you have a problem; it cannot make the problem go away. Low-use outlets during quiet spells are their own special case, covered in Building shutdowns: flushing and monitoring during low use.

One honest caveat

Sampling frequency is a judgement reached through a competent, site-specific risk assessment, not a figure you can lift from a web page. The intervals and action levels that suit a cooling tower, a hospital, a hotel and a small office are not the same, and they shift as your control evidence shifts. Use this to ask sharper questions of your assessment and your service provider, then let the assessment, the system and the people exposed set the actual numbers.

FAQ

Does UK guidance set a fixed Legionella sampling frequency?

Not for most hot and cold water systems. Where temperature is the control and it is holding, routine sampling is generally not expected; where sampling is warranted, the frequency comes from the risk assessment and the type of system rather than a single published figure [2][3].

When should I sample more often than my routine schedule?

After temperature excursions, a positive result, disinfection or remedial works, commissioning, or a change in use, and throughout any outbreak investigation. Those are the moments when extra verification actually earns its cost [4].

Who should take the samples and analyse them?

A competent sampler working to BS 7592 methods, with analysis by a UKAS-accredited laboratory. Where and why each sample is taken matters as much as the lab result, so record the sampling points alongside the numbers [4].

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [4] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [5] HSE, “Control of legionella and other infectious agents in spa-pool systems (HSG282)”. https://www.hse.gov.uk/pubns/books/hsg282.htm

UK guidance on Legionella sampling frequency